Microlens Parallax Measurements with a Warm Spitzer

TL;DR

Warm Spitzer can perform microlens parallax measurements toward the Galactic bulge and Magellanic Clouds, providing insights into lens populations and the disk mass function, especially for faint sources.

Contribution

This paper proposes utilizing warm Spitzer for regular and disruptive microlens parallax observations, expanding capabilities beyond previous measurements with cool Spitzer.

Findings

Potential to observe up to 24 microlens parallaxes per year.

Ability to distinguish lens populations via projected velocity measurements.

Enhanced understanding of the disk mass function, including brown dwarfs.

Abstract

Because Spitzer is an Earth-trailing orbit, losing about 0.1 AU/yr, it is excellently located to perform microlens parallax observations toward the Magellanic Clouds (LMC/SMC) and the Galactic bulge. These yield the so-called ``projected velocity'' of the lens, which can distinguish statistically among different populations. A few such measurements toward the LMC/SMC would reveal the nature of the lenses being detected in this direction (dark halo objects, or ordinary LMC/SMC stars). Cool Spitzer has already made one such measurement of a (rare) bright red-clump source, but warm (presumably less oversubscribed) Spitzer could devote the extra time required to obtain microlens parallaxes for the more common, but fainter, turnoff sources. Warm Spitzer could observe bulge microlenses for 38 days per year, which would permit up to 24 microlens parallaxes per year. This would yield interesting information on the disk mass function, particularly old brown dwarfs, which at present are inaccessible by other techniques. Target-of-Opportunity (TOO) observations should be divided into RTOO/DTOO, i.e., ``regular'' and ``disruptive'' TOOs, as pioneered by the Space Interferometry Mission (SIM). LMC/SMC parallax measurements would be DTOO, but bulge measurements would be RTOO, i.e., they could be scheduled in advance, without knowing exactly which star was to be observed.